1. Field of the Invention
The present invention relates to a method of producing semiconductor devices, and more particularly to a method of selectively forming field isolation regions and active regions in the semiconductor devices.
2. Description of the Prior Art
In the technology of an integrated circuit (IC), in which many active and passive elements and connections are made in or on a semiconductor substrate, a method of forming a field isolation region, i.e. an isolation region for isolating active elements from each other in an integrated circuit, is an important subject of technical development. The field isolation region generally comprises a thick thermal-oxidized layer and is an important factor for determining an area of an integrated circuit chip and a usable area for elements. However, in a case where a thick thermal-oxidized layer is formed, there are problems as to the production time of an integrated circuit, the effects generated by high temperature treatment and the degree of evenness of the surface of the integrated circuit.
One of the conventional methods of forming the above-mentioned field isolation region, comprising a thick oxide layer, is explained with reference to FIGS. 1A and 1B. As illustrated in FIG. 1A, on a single-crystalline silicon substrate 1 there is a silicon layer 2 which is either a surface region of the substrate or a formed epitaxial layer. A patterned silicon nitride film 4, which covers a region 3 for a semiconductor element or a desired circuit element of the silicon layer, is formed by a suitable conventional method. The uncovered portion of the silicon layer 2 is oxidized to form a silicon dioxide layer, i.e. a field oxide layer 5 (FIG. 1B), by a conventional high-temperature oxidizing treatment. The patterned silicon nitride film 4 is removed as illustrated in FIG. 1B. Thereafter, for example, a MOS (metal oxide semiconductor) element or a bipolar transistor is formed at the region 3 of the silicon layer 2 by a conventional method.
In the case of the formation of the MOS element, it is possible to improve characteristics of the MOS element by using a high resistivity silicon epitaxial layer on a low resistivity, i.e. a highly-doped single-crystalline silicon substrate. In this case, for a field oxide layer to attain its function satisfactorily, so-called channel cut regions can be formed by utilizing a phenomenon of impurities diffusing backward from the substrate. However, since the field oxide layer is formed by the conventional field oxidizing treatment at a high temperature ranging from 1000.degree. to 1200.degree. C., preferably about 1100.degree. C., for a period of several hours, impurity diffusion from the silicon substrate into the silicon epitaxial layer (i.e. a so-called auto-doping) is much advanced. Therefore, in expectation of the distances of the diffusion impurities, it is necessary to make the silicon epitaxial layer relatively thick. However, when such a thick silicon epitaxial layer is formed and oxidized, the field oxide layer tends to become thick, which in turn prevents increased circuit density in a semiconductor device.
An object of the present invention is to provide a method of forming a field isolating oxide layer which permits increased circuit density in a semiconductor device, together with a method of forming active regions whose resistivity can be controlled with ease, including intrinsic resistivity regions.
Another object of the present invention is to achieve thermal oxidation of silicon at a lower temperature than that required in a conventional case.